Visible light is a part of electromagnetic spectrum that can be sensed by human eyes. There is not an accurate scope for visible spectrum. In general, the wavelength of electromagnetic wave that can be sensed by human eyes is 400 nm-700 nm. Normal human eyes are most sensitive to electromagnetic wave with the wavelength of about 555 nm, which lies in green band.
Visible light communication technology is a wireless optical communication technology that is developed following the development of light emitting diode (LED) lighting technology. In comparison with traditional wire transmission, visible light communication provides many advantages such as high transmission power, no electromagnetic interference and energy conservation. Therefore it gains widespread attention and research in the world.
LEDs are also being increasingly widely researched and used in various areas. However, it is still hard to create a breakthrough for LEDs in communication technology. Main reasons include the following:
1. Due to the capacity of the LED itself, in case of an input current at a high frequency (>40 MHz), capacitive reactance current created thereby will cause attenuation in the frequency of output signals, which leads to a low response frequency of the device.
2. Patterned substrate has severe absorption to the light.
3. It is hard to obtain a high output power with a single LED chip.
The above aspects lead to low response frequency and low light extraction efficiency of LED that cannot meet the requirement of communication.